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Figure 1. Examples of the relationships between amounts of shoot N fixed (kg N/ha) and shoot dry matter (DM, t/ha) for (a) annual pasture legumes, alfalfa, and pulse crops in SE Australia (Peoples et al. 2001), and (b) winter (lentil, chickpea) and summer (mashbean, soybean) crops growing in different locations in Nepal (Maskey et al. 2001).

Basic improvements in crop agronomy hold great promise for immediate increases in N2 fixation where legumes are already grown (Figure 2). However, just as with inoculation technology, the prospects for such increases must be considered within the context of the present constraints to the adoption of existing knowledge by farmers. For example, research has demonstrated that nutritional deficiencies induced by poor P supply or soil acidity commonly restrict legume growth and BNF (Giller, Cadisch 1995; Peoples et al. 1995). Yet the implementation of the simple fertilizer strategies needed to ameliorate these soil limitations may require economic, transport and knowledge barriers to be removed. Other research has indicated relationships between biomass production, rates of BNF and legume population (Figure 3), and poor legume density and vigor have been implicated as important factors contributing to low inputs of fixed N in farmers' fields in South Asia (Maskey et al. 2001). However, access to better quality seed to ensure good germination and greater tolerance to pests and diseases is likely to be limited for resource poor farmers.

Even in the absence of access to inoculants, fertilizers or quality seed there still appears to be considerable potential for enhancing BNF inputs in farming systems through the inclusion of more legumes in farming systems (Figure 2). The small area of land cropped with legumes has been identified as a major constraint to N2 fixation in Africa (Giller et al. 2000), but this can be considered to be a common factor since few countries have cropping ratios of cereal-.legume less than 10:1. The ratio of land area seeded to cereals versus pulse crops has not changed in 40 years, whereas the productivity of cereals relative to pulse crops has increased significantly (Figure 4). Increased areas of legumes might be achieved by including more leguminous pastures or crops in rotations, the wider use of intercropping or cover crop strategies where legumes and non-legumes are grown together, or the introduction of legumes to areas of fallowed or degraded land. Legume green manures and agroforestry systems are other strategies that can potentially enhance inputs of fixed N (e.g. Peoples et al. 1996). However, they are not without limitations that restrict their wide spread use (Giller et al. 2000). Changes to common practices by Asian farmers so that legume residues are retained rather than removed after grain harvest would also have a huge impact on the subsequent benefits of fixed N to following crops. On-farm data from South Asia suggests that the return of up to 50 kg fixed N/ha is commonly forgone when above-ground vegetative residues are removed along with the grain.

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Rhizobial strain selection

Changes to farming systems

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• Introducing legumes to new areas

• Retaining legume residues implementation of existing knowledge

• Amelioration of nutritional constraints

• Inoculation

• Best-practice crop agronomy Current leva!

Research effort (years)

Figure 2. Conceptual representation of possible approaches to increase inputs of fixed N and the potential time scale of benefits to agriculture. Modified from Giller and Cadisch (1995).

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Figure 3. On-farm data collected from Vietnam that illustrates the relationship between plant population and N2 fixation.

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Figure 3. On-farm data collected from Vietnam that illustrates the relationship between plant population and N2 fixation.

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Figure 4. Global area under production (a) and production levels (b) of cereal and pulse crops (FAO 2001).

5.2. Genetic improvements. Further increases in the potential inputs of fixed N might require selection of better adapted rhizobial strains and/or genetic improvements through plant breeding (Giller, Cadisch 1995; Herridge, Rose 2000). In the long-term the use of modern molecular techniques to transfer of the capacity to fix N to non-legume crops could be imagined to have benefits to agriculture, but there are enormous physiological and biochemical obstacles that will need to be overcome before this goal can be realized (Ladha, Reddy 2000). There is also a concern that if cereals could fix their own N that this may encourage an increased reliance upon monocultures rather than rotations with legumes and other crops which may bring with it other problems for agriculture (Giller, Cadisch 1995). Ultimately the gains in amounts of N2 fixed likely to be gained through genetics will be far more modest than the adoption of current agronomic knowledge (Figure 2).

6. Conclusions

Globally, legumes in farming systems should routinely be fixing >100 kg N/ha/year, but in reality they don't. A major reason for this is that the relevant technology is either not in the hands of the fanners, or they cannot adopt it because of economic or operational imperatives. These issues have been raised before (Giller, Cadisch 1995), but the ability to overcome constraints at the farm level, or to undertake applied BNF research that will be of direct benefit to farmers continues to deteriorate rather than improve. The poor countries become poorer, the training and support programs like NifTAL that were instrumental in advancing BNF in the past (particularly in the developing world) have been wound up or redirected, and BNF receives little attention from the CGIAR institutes. There is a need for some strong policy intervention to redress this trend. The recent initiative such as the FAO-sponsored meeting during March 2001 in Rome on BNF is an encouraging step in the right direction.

7. References

FAO (2001) FAOSTAT Agricultural Database, http://apps.fao.org/ Giller KE, Cadisch G (1995) Plant Soil 174, 255-277

Giller KE et al. (2000) In Pedrosa FO et al. (eds), Nitrogen Fixation: From Molecules to Crop

Productivity, pp. 525-530, Kluwer Academic Publ., Dordrecht, The Netherlands Herridge D, Rose I (2000) Field Crops Res. 65, 229-248 Kelner et al. (1997) Agric. Ecosystem Environ. 64,1-10

Ladha JK, Reddy P (2000) The Quest for Nitrogen Fixation in Rice, IRRI, Los Banos, Philippines, 354 pp.

Ledgard ST, Giller KE (1995) In Bacon PE (ed), Nitrogen Fertilization in the Environment, pp. 443-486, Marcel Dekker Inc., New York Maskey SL et al. (2001) Field Crops Res. 70, 209-221 Peoples MB, Craswell ET (1992) Plant Soil 141, 13-39

Peoples MB, Herridge DF (2000) In Pedrosa FO et al. (eds), Nitrogen Fixation: From Molecules to

Crop Productivity, pp. 519-524, Kluwer Academic Publ., Dordrecht, The Netherlands Peoples MB et al. (1995) Plant Soil 174, 83-101 Peoples MB et al. (1996) Plant Soil 182,125-137 Peoples MB et al. (2001) Plant Soil 228, 29-41 Unkovich MJ, Pate JS (2000) Field Crops Res. 65, 211-228 van Kessel C, Hartley C (2000) Field Crops Res. 65, 165-181

8. Acknowledgements

The financial support of the Australian Centre for International Agricultural Research (ACIAR) and NSERC (Canada) is gratefully acknowledged, as is the cooperation of our many collaborators throughout the world.

Poster Papers

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